A Janus-type Doppler ground-speed measuring apparatus has been well known as an apparatus for measuring ground speeds of vehicles (e.g. “Microwave Front End for True Ground Speed Measurements” by J. Kehrbeck et al., Journal of Navigation, pp. 88-96, Vol. 48, No. 1, 1995). The Janus-type system has two Doppler ground-speed measuring apparatus in the fore and aft directions of a vehicle to reduce deterioration in precision of detection due to assembling errors, road conditions, and unbalancing of loads in the fore and aft directions.
Japanese Application Patent Laid-open Publication No. Hei 11-352225 discloses a ground speed measuring apparatus that enables measurement of speeds in fore, aft, and athwartship directions of the vehicle by employing three radio beams originated from transmitting means which are disposed on vertices of a certain equilateral triangle.